In mobile phones, a typical application, a battery pack is provided to supply power to the components. However, mobiles phones can contain several sub-circuits which each require unique voltage levels different from those supplied by the battery pack (sometimes higher or lower than the battery voltage, or even negative voltage). Additionally, the battery voltage declines as its stored power is drained. DC-to-DC converters offer a method of generating multiple controlled voltages from a single variable battery voltage, thereby saving space instead of using multiple batteries to supply different parts of the device.
There are many types of DC-to-DC converters available, depending on the particular function required. Two of the most common are a boost converter, which is used to increase the voltage level, and a buck converter, which is used to decrease the voltage level. Both of these are switched-mode converters, which perform the conversion by applying a DC voltage across an inductor or transformer for a period of time (usually in the 100 kHz to 5 MHz range). This causes current to flow through the inductor, or transformer, which stores energy magnetically. The voltage is then switched off thus causing the stored energy to be transferred to the voltage output in a controlled manner. By adjusting the ratio of on/off time, the output voltage can be regulated even as the current demand changes.
FIG. 1 shows a circuit diagram of a simple Boost DC-to-DC converter. The circuit contains a power supply 1, a high frequency switch 2, a diode 3, an inductor 4 and a load resistor 5.
Present technology uses electrolytic, tantalum or ceramic capacitors for output voltage filtering for these kinds of power management application specific integrated circuits (ASIC). The purpose of such capacitors is to store the energy that is output in the form of pulses by the DC/DC converter, and to further smooth the output voltage signal by removing noise and other high frequency elements.
One particular sub-circuit, which requires a unique voltage, is that used to supply power to an LED string, as is generally used to backlight the LCD screen of the mobile telephone. Occasionally the backlights are required to be dimmed, such as when the backlight is switched on or off, or when the light may need to be switched to a lower level for usability or power saving purposes. This can also be utilised with an ambient light sensor. In order to achieve the dimming effect, the output of the DC-to-DC switched-mode converter is modulated using pulse width modulation (PWM). Further, to achieve power saving, the DC-to-DC converter may be disabled, or partially disabled, during the OFF time of the PWM cycle.
FIG. 2 shows a block diagram of a typical circuit that is currently employed to provide power to a dimmable LED string for a backlight. The circuit contains a PWM control signal generator 6, a DC-to-DC switched-mode converter 7 including an enable switch 71, a battery pack 8, an output filter capacitor 9 and an LED string 10.
PWM uses a rectangular wave signal and modulates the pulse width of the rectangular wave signal, such that the ratio between the ON time (the time for which the signal is asserted) and the time period of the signal varies according to the required operation of the LED string. FIG. 3 shows a PWM signal which, when applied as a PWM control signal to the DC-to-DC switched-mode converter 7 and LED string 10, would have the effect of gradually dimming the LED string 10. The figure shows the varying duty cycle 21 and the time period 20.
The PWM signal is applied as a PWM control signal to the DC-to-DC switched-mode converter 7 and controls an integrated enable switch 71 such that the enable switch 71 is closed when the PWM control signal is asserted, and the enable switch 71 is open when the PWM control signal is not asserted. The DC-to-DC switched-mode converter only outputs a converted voltage to the LED string when the enable switch is closed. Therefore, a PWM voltage signal, with the same frequency and modulation as the PWM control signal, is output by the DC-to-DC switched-mode converter. Subsequently the LED string will flicker between emitting light when the PWM output voltage signal is asserted and not emitting light when the PWM output voltage signal is not asserted. If the PWM control signal of FIG. 3 were applied to the DC-to-DC switched-mode converter 7, the LED string 10 would, at first, emit light for the majority of the time period 20 of the PWM control signal. Over time, however, the proportion of the time period 20 for which the LED string 10 emits light gradually decreases. Therefore, if the frequency of the PWM control signal is appropriate, the light emitted by the LED string 10 would be appear, to the human eye, to be getting dimmer.
The frequency, in order that the flickering of the LED string is not detectable, must be above about 50 Hz (e.g. 300 Hz). These frequencies however are in the audible frequency range. Ceramic filtering capacitors tend to display piezoelectric qualities and therefore, when the PWM output voltage signal is at an audible frequency, a buzzing noise is emitted by the capacitor as a result of its continuous deformation and reformation. This noise is not acceptable.
A further problem of the device shown in FIG. 2 is that DC-to-DC switched-mode converters, operating with an inductor, draw the most current when switching on and the filtering capacitor is charged from a discharged, or partially discharged, state. This is particularly important when using PWM modulation, as the circuit is switched on and off a few hundred times every second and thus the increased current requires that the inductor and other components be sized according to the peak currents encountered, and therefore increase the size and price of such components, or increase their operating stress levels.
A further problem of such a circuit is that the extra current causes increased disturbance to the power line that supplies the circuit, possibly resulting in interference with other devices, or the emission of electromagnetic radiation that exceeds regulatory limits.
Similar problems arise in any DC supply technology that is disabled or powered off during the de-asserted state of a PWM signal waveform such that the output voltage is undriven in the OFF state; the voltage on the capacitor is the only determinant for when acoustical noise will be evident. Even in the case where a DC supply is the driving source, the output capacitor voltage fluctuation between the ON and OFF states is the source of the acoustical noise. Depending on whether current limiting is applied on start-up, the capacitor still appears as a very low impedance load when the power supply is re-enabled, which can cause inrush current spikes, as well. If the power supply is current-limited, the increased rise time of the voltage on the output capacitor causes significant non-linearity in the output voltage/current at low duty cycles of the PWM waveform.
One possibility used to overcome the problem of the audible buzzing of the capacitor is the use of tantalum capacitors instead of ceramic capacitors. However, although these have better resonance characteristics, they are also much more expensive and have other drawbacks such as having high effective series resistance (ESR). A further possibility is to modulate the DC current through the LED string, instead of using PWM, though this can lead to a colour change, as white LEDs (which are typically used to backlight colour LCD screens) change colour with the supply current, and the characteristics of LEDs may vary, relative to one another, at currents that are significantly less than, or greater than, the current at which they are characterised.